Petrology of Mafic and Intermediate-Composition Magmas Associated with Rhyolite Ash-flow Volcanism, Sierra Madre Occidental

本研究探讨了基性岩浆和中间成分岩浆在托莫奇火山中心（TVC）流纹岩成因中的作用。托莫奇火山中心的地层剖面最为完整，火山活动的历史在西马德雷山脉已知的喷发中心中得到了最好的记录。最近在TVC进行的野外和年代学研究表明，两种不同的岩性，即斑岩型双辉石安山岩和贫晶玄武岩安山岩，是形成大体积流纹岩灰流喷发的岩浆系统的重要组成部分。这些岩性和流纹岩之间的遗传关系是允许的，但现有的数据很少约束。本研究的主要目标是通过以下方式进一步评估这种关系：(a)所有岩石类型的矿物学、同位素（Sr、Nd）和地球化学变化的比较和建模；(b)混合崩塌后熔岩的岩石学研究，以表征拉斯瓦拉斯破火山口下储层的深层基性部分；与挤压玄武岩安山岩比较。研究结果将限制流纹岩的成因模型，以及在被认为是世界上最大的“流纹岩为主”的火山场中相关的、更基性的岩浆。研究结果对于建立火山活动类型与构造机制的联系模型，以及我们对与大体积破火山口下储层演化相关的过程的理解也很重要。

This study addresses the role of mafic and intermediate- composition magmas in the genesis of rhyolites at the Tomochic volcanic center (TVC), where the stratigraphic section is the most complete and where the history of volcanism is best documented among known eruptive centers in the Sierra Madre Occidental. Recent field and chronologic work at the TVC has indicated that two distinct lithologies, porphyritic two-pyroxene andesite and crystal-poor basaltic andesite, were important components of the magma systems that fed large-volume rhyolite ash-flow eruptions. A genetic relationship between these lithologies and the rhyolites is allowed, but is poorly constrained, by available data. The principal goal of this study will be to evaluate that relationship further, via: (a)comparison and modeling of mineralogic, isotopic (Sr, Nd), and geochemical variations among all rock types, and (b) petrologic study of hybrid post-collapse lavas, to characterize the deep, mafic parts of the reservoir beneath the Las Varas caldera, for comparison with extruded basaltic andesite. Results will constrain model pertaining to the genesis of rhyolites, and of associated, more mafic magmas, in what is considered the world's largest "rhyolite-dominated"volcanic field. Results will also be important to models linking style of volcanism with tectonic regime, and to our understanding of processes associated with the evolution of large-volume subcaldera reservoirs.